Simians immunized with human lymphoid cell populations which are entirely "T" or "B" in terms of function or membrane characteristics are being characterized and the various serological specificities defined. Simian antisera which are specific for human thymocytes or peripheral blood "T" lymphocytes have been described. Similar attempts are being made to define "B" lymphocyte specific antigens. The thymocyte and T lymphocyte specific antisera will be used to isolate and biochemically characterize the specific antigens with which they react.